# -*- coding: utf-8 -*-
"""
Created on Wed Sep 27 13:35:44 2017

@author: Luther
"""

score = int(input('请输入分数：'))
if score > 100:
    print('Invalid score')
elif score >= 90:
    print('A')
elif score >= 70:
    print('B')
elif score >= 60:
    print('C')
elif score >= 0:
    print('D')
else:
    print('Invalid score')
